Benchmark
Definition
A benchmark is a permanent or semi-permanent reference point established at a specific location with a precisely determined elevation. In surveying, benchmarks serve as fundamental control points for vertical measurements and are essential for establishing consistent height references across surveying projects.
Historical Background
The term "benchmark" originated from the practice of surveyors marking a small horizontal line or "bench" on structures, creating a precise point upon which a surveying rod could rest. This practice became formalized as the science of surveying advanced, particularly during the 19th century when national survey programs required standardized elevation references.
Types of Benchmarks
Permanent Benchmarks
These are established on stable structures such as courthouse steps, bridge abutments, or concrete monuments that are expected to remain undisturbed for decades or centuries. They are typically marked with brass plaques or metal tablets.Temporary Benchmarks
These are created during specific surveying projects and are intended to exist only for the duration of the work. They may be marked with painted lines or temporary monuments.Monumented Benchmarks
These include physical markers such as concrete posts, iron pipes, or brass discs that clearly identify the benchmark location.Purpose and Application
Benchmarks serve multiple critical functions in surveying:
1. Vertical Control: They establish a network of known elevations that surveyors can reference for accurate vertical measurements.
2. Leveling Operations: Benchmarks provide starting and ending points for leveling surveys, allowing surveyors to check their work and maintain accuracy.
3. Grade Establishment: Construction projects rely on benchmarks to establish proper grades and elevations for buildings, roads, and utilities.
4. Subsidence Monitoring: Repeated measurements between benchmarks can detect ground settlement or elevation changes over time.
5. Data Standardization: Benchmarks create a common reference system, ensuring consistency across multiple projects and surveyors.
Elevation Standards
In many countries, national surveying agencies maintain networks of benchmarks tied to a national vertical datum. The elevation of each benchmark is precisely documented and published in official records. Surveyors can obtain this information to establish accurate references for their projects.
Benchmark Identification
Each benchmark is typically assigned a unique identifier and documented with:
Modern Technology
While traditional spirit leveling remains important, modern benchmarking increasingly incorporates:
Importance in Surveying Practice
Benchmarks are indispensable to surveying because they:
Preservation and Maintenance
Benchmarks require careful preservation. Surveyors must avoid disturbing benchmark markers and should report any damaged or missing benchmarks to relevant authorities to maintain the integrity of vertical control networks.